Regulation of transport of nucleic acid precursors is well correlated with growth control phenomena in both normal and oncogenically transformed animal cell systems. In order to study the precise biochemical mechanisms of transport, including the enzymology and metabolic pathways, in vitro, mutants deficient in enzymes for the initial purine bases and nucleosides will be obtained. Three new classes of animal cell mutants which lack either purine nucleoside phosphorylase, adenosine deaminase, or 5' nucleotidase will be selected and analyzed for regulation, pathway and mechanism of transport in order to better elucidate the structure and function of cancer cells and normal cells. Mutants for pleiotropic resistance to drugs involving a permeability change will be investigated for specific changes in transport systems and susceptibility to chemical carcinogenesis.